Hello Amal!

The solution for your question is as follows:-

No. of moles (n) in 0.05 g of water= 0.05/18 ( since the molecular mass of water is 18 g)

Therefore, n= 0.00277777778    --(1)

Also, you know that no. of molecules in 1 mole h2o = 6.023×10^23 (Avogadro Number)

So, the required no. of molecules in n moles of h2o= 6.023×10^23 × 0.00277777778

= 1.67 ×10^21